The electrophoretic effect is well known and the prior art is replete with a number of patents and articles which describe the effect. As will be recognized by a person skilled in the art, the electrophoretic effect operates on the principle that certain particles, when suspended in a medium, can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. Electrophoretic image displays (EPIDs) utilize the electrophoretic effect to produce desired images. In prior art EPIDs colored dielectric particles are suspended in a fluid medium that is either clear or an optically contrasting color as compared to the dielectric particles. The colored electrophoretic particles are then caused to selectively migrate to, and impinge upon, a transparent screen electrode, thereby displacing the fluid medium from the screen and creating the desired image.
As will be recognized by a person skilled in the art, the selection of the electrophoretic particles used in the EPID is very important in determining the performance of the EPID and the quality of the viewed image produced. Ideally, electrophoretic particles should have an optimum charge/mass ratio, which is dependent upon the particle size and surface charge, in order to obtain good electrostatic deposition at high velocity as well as rapid reversal of particle motion when voltages change. Additionally, it is desirable to utilize electrophoretic particles that have essentially the same density as the fluid medium in which they are suspended. By using electrophoretic particles of essentially the same density as the suspension medium, the migration of the electrophoretic particles through the medium remains independent of both the orientation of the EPID and the forces of gravity.
To effect the greatest optical contrast between electrophoretic particles and the suspension medium, it is desirable to have either light-colored particles suspended in a dark medium or black particles suspended in a backlighted clear medium. In the prior art, it has been proven difficult to produce black electrophoretic particles that are dielectric, of uniform size and have a density matching that of a common suspension medium. As a result, EPIDs, commonly use readily manufactured light colored electrophoretic particles suspended in dark media. Such EPIDs are exemplified in U.S. Pat. Nos. 4,655,897 to DiSanto et al., 4,093,534 to Carter et al., 4,298,448 to Muller et al., and 4,285,801 to Chaing. In such art, the light colored particles are commonly inorganic pigments.
Titanium dioxide has been used in EPIDs to produce a good optical contrast between the white particles and the colored suspension medium, however, it has a density about 4 g/m.sup.3 which is too high to match with any organic liquid to prevent the sedimentation problem. In the past decade, great effort has been made to solve the density problem of titanium dioxide. However, very little work has succeeded without trading off the quality of the images, especially in regard to the whiteness. Coating titanium dioxide particles with a polymeric material to reduce the density of titanium dioxide is an example.
The development of suitable dielectric black particles has remained a long felt need in the art of electrophoretic image displays. In am other than EPIDs, black particles are commonly produced from carbon black. However, carbon blacks are not readily adaptable to EPIDs because carbon blacks are conductive and the density of carbon black is not readily matched to a suitable suspension medium. Research efforts have been made in an attempt to solve the density and conductivity problems of carbon blacks, however, none has succeeded without trading off the blackness of the particles created. Such efforts to produce dielectric particles from carbon blacks are exemplified in the following articles: Fowkes et al. "Electrophoretic Display Medium", a research project report for the Department of Chemistry of Lehigh University (Aug. 28, 1989) and Hou et al. "Polymer-Encapsulated Particles With Controlled Morphologies", Ph.D. Dissertion (LeHigh University, 1991).
It is therefore an object of the present invention to produce light and dark dielectric particles suitable for use in EPIDs. It is a further object to produce colored dielectric particles which may be used in EPIDs.